Page:Encyclopædia Britannica, Ninth Edition, v. 17.djvu/344

330 NEMEETINES Jff Hoplonemertines it is smaller and rounded ; in Malacobdclla and Akrostomum it, moreover, serves for the extrusion of the proboscis, which emerges by a separate dorsal opening just inside the mouth. The cesophagus is the anterior portion of the digestive canal; its walls are folded longitudinally, comparatively thick, and provided with longitudinal muscular fibres. Two layers are specially obvious in its walls, the inner layer bordering the lu men being composed of smaller ciliated cells, the outer thicker one containing numerous granular cells and hav ing a more glandular character. Outside the Avail of the cesophagus a vascular space has been detected (11) which is in direct continuity with the longitudinal blood-vessels. In cer tain cases, however, the walls of the cesophagus appear to be very closely applied to the muscular body-wall, and this vas cular space thereby con siderably reduced. Lff The posterior portion of the intestine is speci ally characterized by the appearance of the intes tinal diverticula hori zontally and symmetric ally placed right and left and opposite to each other. Sometimes this re gion, into which the oeso phagus leads, stretches forwards under the cesophagus (Hoplone mertines) for a certain distance, anteriorly ter minating by a cul-de- sac. Cases of asym metry or irregularity in the arrangement of the caeca, though sometimes occurring, are not nor mal. At the tip of the Fig. 17. tail, where the growth FIGS. 15-17. Diagrammatic sections to show dis- of the animal takes position of internal organs in Carinella (Palxo- fhp f^po arp al nemertea), fig. 15, Schizonemertea, fig. 16, and Iloplonemertea, fig. !17. C, cellular portion of integument ; B, basement membrane ; A, circu lar muscular layer; A , longitudinal do.; A", second circular (in Carinella); A ", second longi tudinal (in Schizonemertea) ; JV, nervous layer ; LN, lateral nerves ; PS, cavity of proboscidian sheath (the sheath itself of varying thickness) ; P, proboscis ; /, intestine ; LBv, lateral blood vessel ; JJBv, dorsal do. ; CT, connective tissue. LN- C place, ways eminently regular. So they are throughout the whole body in most of the Hoplonemertines. In Carinella they are generally deficient and the intestine straight ; in young specimens of this species, however, they occur, though less regular and more in the form of incipient foldings by which the digestive surface is increased. The inner surface of the intestinal cajca is ciliated, the caeca themselves are sometimes especially in the hindermost portion of the body of a considerably smaller lumen than the intermediate genital spaces ; sometimes, however, the reverse is the case, and in both cases it is the smaller lumen that appears enclosed between and suspended by the transverse fibres constituting the muscular dissepiments above mentioned. The anus is situated terminally, the muscular body-wall through which the intestine must find its way outwards probably acting in this region the part of a sphincter. The lateral nerve stems mostly terminate on both sides in closest proximity to the anus ; in certain species, however, they interfuse by a transverse connexion above the anus. The longitudinal blood-vessels do the same. The question has been raised whether the regular intestinal caeca of Nemertines might not be compared with those intestinal diverticula of the embryo Amphioxus which ultimately become the mesoblastic somites of the adult (8). This view would be a further extension of the views concerning the ccelom first propounded by Huxley. (/) Circulatory Apparatus. This consists of three longitudinal trunks, a median and two lateral ones. They are in direct con nexion with each other both at the posterior and at the anterior end of the body. At the posterior end they communicate together by a T-shaped connexion in a simple and uniform way. Anteriorly there 13 a certain amount of difference in the arrangement. "Whereas in the Hoplonemertines an arrangement prevails as represented in the paratus in the ante- nor body-region of a Hoplonemertine. fig. 18, the lateral stems in the Schizonemertines, while entirely uniform all through the posterior portion of the body, no longer individually exist in the cesophageal region, but here dissolve themselves into a network of vascu lar spaces surrounding this portion of the di gestive tract (11). The median dorsal vessel, however, remains distinct, but instead of con tinuing its course beneath the proboscidian sheath it is first enclosed by the ventral muscu lature of this organ, and still farther forwards it even bulges out longitudinally into the cavity of the sheath. Anteriorly it finally communi cates with the lacunae just mentioned, which surround the cesophagus, bathe the posterior lobes of the brain, pass through the nerve ring together with the proboscidian sheath, and are generally continued in front of the brain as a lacunar space in the muscular tissue, one on each side. Special mention must bo made of the delicate transverse vessels regularly connecting the longitudinal and the lateral ones. They are metamerically placed, and belong to the same metamer as the digestive coeca, thus alternating with the generative sacs. The blood fluid does not flow in any definite direction ; its movements are largely influenced by those of the muscular body-wall. It is colourless, and contains definite corpuscles, which are round or elliptical, and in many Hoplonemertines are coloured red by haemo globin, being colourless in other species. The circulatory system of Carinella is considerably different, being more lacunar and less restricted to definite vascular channels. Two lateral longitudinal lacunas form, so to say, the forerunners of the lateral vessels. A median longitudinal vessel and transverse connecting trunks have not as yet been detected. There are large lacunas in the head in front of the ganglia. (g) Nephridia. Although these organs were already very well known to Max Schultze (14), their presence in Nemertines was repeatedly and seriously disputed until Yon Kennel (10) definitely proved their existence and gave details concerning their histology. With the exception of a few genera where they have not as yet been discovered (Carinella), one pair of nephridia appears to be very generally present. They essentially consist of a complex coiled tube, one on each side of the cesophagus (fig. 1), communicating with the exterior by a duct piercing the body-wall. The two openings of the nephridia are situated sometimes more towards the ventral, at other times more towards the dorsal side. Even in the larger Schizonemertines these pores are only a few millimetres behind the mouth region. Internal funnel-shaped openings, although sought for, have as yet not been detected. The coiled tubes extend both forwards and backwards of the external opening, by far the greater portion being situated backwards. The anterior coils reach forwards till in the immediate vicinity of the posterior brain-lobes. The coils are tubiform, with an internal lumen, only one layer of rather large cells constituting the walls. These cells are ciliated ; in some transparent species the internal ciliary move ment can be observed during lite. In transverse sections the nephridia can be shown to be generally situated in the region limited by (1) the proboscidian sheath, (2) the upper wall of the intestine, (3) the muscular body- wall. No trace of nephridia is found posterior to the cesophagus. (h) Generative System. In the Nemertines the sexes are separate, with only very few exceptions (12) (Tdrastcmma liermapliroditica, Marion). The generative products are contained in separate pouches placed metamerically in the way noticed above in treating of the digestive system. They are conveyed outwards along narrow canals, one pair for each metamer piercing the muscular body-wall, and visible on the outside in mature individuals as minute light- coloured specks. The ova and spermatozoa, when mature, present no peculiarities. As the ova are in many species deposited in a gelatinous tube secreted by the body-walls, in which they are arranged (three or more together) in flask-shaped cavities, impreg nation must probably take place either before or at the very moment of their being deposited. The exact mode has not yet been noticed. Another point not yet sufficiently settled is the oogenesis in Nemertines. In several cases the ova appear to originate directly as the lining of the generative pouches, but the exact part which the mesoblastic connective tissue plays, both with regard to these pouches and to the generative products themselves, remains yet to be settled. Prosorhochmus claparedii is a viviparous form. DEVELOPMENT. The embryology of the Nemertines offers Develop- some very remarkable peculiarities. Our knowledge of m ent. the development of the most primitive forms is very scanty. Of that of Carinella absolutely nothing is known. On Cephalothrix we have observations, in certain respects con- tradictory. Both Schizo- and Hoplonemertea have been more exhaustively studied, the first, as was noticed above,